Methods and system for control of a two-stage turbocharger

ABSTRACT

Various methods and systems are provided for regulating air flow through a two-stage turbocharger. In one embodiment, a method for an engine comprises, in response to a transient engine operating event, adjusting a provided amount of exhaust gas recirculation provided to an intake of the engine based on at least one of desired intake oxygen, one or more turbocharger conditions of a turbocharger, or a magnitude of the transient engine operating event.

FIELD

Embodiments of the subject matter disclosed herein relate to an engine,engine components, and an engine system, for example.

BACKGROUND

Turbocharging an engine allows the engine to provide power similar tothat of a larger displacement engine. Thus, turbocharging can extend theoperating region of an engine. Turbochargers function by compressingintake air in a compressor via a turbine operated by exhaust gas flow.Under certain conditions, the flow rate and pressure ratio across theturbocharger can fluctuate to levels that may result in noisedisturbances, and in more severe cases, performance issues andcompressor or turbine degradation.

Such turbocharger performance issues may be mitigated by adjusting theflow rate through the turbocharger, such as by adjusting one or moreturbocharger bypass valves. However, such adjustments may not provideadequate avoidance of the flow rate/pressure ratio fluctuations, or maycompromise power, fuel economy, and/or emissions.

BRIEF DESCRIPTION

In one embodiment, a method for an engine comprises, in response to atransient engine operating event, adjusting a provided amount of exhaustgas recirculation provided to an intake of the engine based on at leastone of desired intake oxygen, one or more turbocharger conditions of aturbocharger, or a magnitude of the transient engine operating event.

In this way, the amount of exhaust gas recirculation provided to theintake may be adjusted during a transient event to maintain optimal airflow through the turbocharger while still accounting for the desiredintake oxygen amount to control emissions. In one example, the transientevent may be an increase in engine load, and the amount of exhaust gasrecirculation provided to the intake may be increased in order to lowerthe mass air flow through the turbocharger and avoid turbocharger choke.

It should be understood that the brief description above is provided tointroduce in simplified form a selection of concepts that are furtherdescribed in the detailed description. It is not meant to identify keyor essential features of the claimed subject matter, the scope of whichis defined uniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood from reading thefollowing description of non-limiting embodiments, with reference to theattached drawings, wherein below:

FIG. 1 shows a schematic diagram of a rail vehicle with an engineaccording to an embodiment of the invention.

FIG. 2 is a flow chart illustrating a method for controlling an exhaustgas recirculation amount according to an embodiment of the invention.

FIG. 3 is a diagram illustrating a control routine for determining themagnitude of a transient event according to an embodiment of theinvention.

FIG. 4 is a diagram illustrating a control routine for correcting aturbocharger speed according to an embodiment of the invention.

FIG. 5 is a diagram illustrating a control routine for determining anoptimal air flow amount through two stages of a turbocharger accordingto an embodiment of the invention.

FIG. 6 is a diagram illustrating a control routine for determining anamount of EGR according to an embodiment of the invention.

FIG. 7 is a turbocharger flow map for a high-pressure turbochargeraccording to an embodiment of the present invention.

FIG. 8 is a turbocharger flow map for a low-pressure turbochargeraccording to an embodiment of the present invention.

DETAILED DESCRIPTION

The following description relates to various embodiments for regulatingair flow through a two-stage turbocharger. During transient events,sudden changes in engine speed and load may result in air flowfluctuations. During an increase in engine load, for example, the amountof exhaust gas recirculated back to the intake may decrease, thussubjecting the turbocharger to additional air flow beyond the toleranceof the turbocharger. Such turbocharger over speed events, also referredto as turbocharger choke, may cause degradation to the turbocharger. Toavoid turbocharger choke during a transient event, exhaust gasrecirculation (EGR) to the intake of the engine may be adjusted in orderto change the mass air flow through the turbocharger. For example, theEGR may be increased to decrease the turbocharger mass air flow. Theadjustment to the provided EGR amount to provide optimal turbochargerair flow may be balanced with an EGR amount that maintains exhaustemissions at a desirable level. In this way, turbocharger over speed maybe avoided during transient events.

The approach described herein may be employed in a variety of enginetypes, and a variety of engine-driven systems. Some of these systems maybe stationary, while others may be on semi-mobile or mobile platforms.Semi-mobile platforms may be relocated between operational periods, suchas mounted on flatbed trailers. Mobile platforms include self-propelledvehicles. Such vehicles can include mining equipment, marine vessels,on-road transportation vehicles, off-highway vehicles (OHV), and railvehicles. For clarity of illustration, a locomotive is provided as anexample of a mobile platform supporting a system incorporating anembodiment of the invention.

Before further discussion of the approach for regulating turbochargerair flow, an example of a platform is disclosed in which the enginesystem may be installed in a vehicle, such as a rail vehicle. Forexample, FIG. 1 shows a block diagram of an embodiment of a vehiclesystem 100 (e.g., a locomotive system), herein depicted as a railvehicle 106, configured to run on a rail 102 via a plurality of wheels110. As depicted, the rail vehicle 106 includes an engine 104. In othernon-limiting embodiments, the engine 104 may be a stationary engine,such as in a power-plant application, or an engine in a marine vessel oroff-highway vehicle propulsion system as noted above.

The engine 104 receives intake air for combustion from an intake, suchas an intake manifold 115. The intake may be any suitable conduit orconduits through which gases flow to enter the engine. For example, theintake may include the intake manifold 115, the intake passage 114, andthe like. The intake passage 114 receives ambient air from an air filter(not shown) that filters air from outside of a vehicle in which theengine 104 may be positioned. Exhaust gas resulting from combustion inthe engine 104 is supplied to an exhaust, such as exhaust passage 116.The exhaust may be any suitable conduit through which gases flow fromthe engine. For example, the exhaust may include an exhaust manifold117, the exhaust passage 116, and the like. Exhaust gas flows throughthe exhaust passage 116, and out of an exhaust stack of the rail vehicle106. In one example, the engine 104 is a diesel engine that combusts airand diesel fuel through compression ignition. In other non-limitingembodiments, the engine 104 may combust fuel including gasoline,kerosene, biodiesel, or other petroleum distillates of similar densitythrough compression ignition (and/or spark ignition).

In one embodiment, the rail vehicle 106 is a diesel-electric vehicle. Asdepicted in FIG. 1, the engine 104 is coupled to an electric powergeneration system, which includes an alternator/generator 140 andelectric traction motors 112. For example, the engine 104 is a dieselengine that generates a torque output that is transmitted to thealternator/generator 140 which is mechanically coupled to the engine104. The alternator/generator 140 produces electrical power that may bestored and applied for subsequent propagation to a variety of downstreamelectrical components. As an example, the alternator/generator 140 maybe electrically coupled to a plurality of traction motors 112 and thealternator/generator 140 may provide electrical power to the pluralityof traction motors 112. As depicted, the plurality of traction motors112 are each connected to one of a plurality of wheels 110 to providetractive power to propel the rail vehicle 106. One example configurationincludes one traction motor per wheel. As depicted herein, six pairs oftraction motors correspond to each of six pairs of wheels of the railvehicle. In another example, alternator/generator 140 may be coupled toone or more resistive grids 142. The resistive grids 142 may beconfigured to dissipate excess engine torque via heat produced by thegrids from electricity generated by alternator/generator 140.

In the embodiment depicted in FIG. 1, the engine 104 is a V-12 enginehaving twelve cylinders. In other examples, the engine may be a V-6,V-8, V-10, V-16, I-4, I-6, I-8, opposed 4, or another engine type. Asdepicted, the engine 104 includes a subset of non-donor cylinders 105,which includes six cylinders that supply exhaust gas exclusively to anon-donor cylinder exhaust manifold 117, and a subset of donor cylinders107, which includes six cylinders that supply exhaust gas exclusively toa donor cylinder exhaust manifold 119. In other embodiments, the enginemay include at least one donor cylinder and at least one non-donorcylinder. For example, the engine may have four donor cylinders andeight non-donor cylinders, or three donor cylinders and nine non-donorcylinders. It should be understood, the engine may have any desirednumbers of donor cylinders and non-donor cylinders, with the number ofdonor cylinders typically lower than the number of non-donor cylinders.

As depicted in FIG. 1, the non-donor cylinders 105 are coupled to theexhaust passage 116 to route exhaust gas from the engine to atmosphere(after it passes through an exhaust gas treatment system 130 and firstand second turbochargers 120 and 124). The donor cylinders 107, whichprovide engine exhaust gas recirculation (EGR), are coupled exclusivelyto an EGR passage 162 of an EGR system 160 which routes exhaust gas fromthe donor cylinders 107 to the intake passage 114 of the engine 104, andnot to atmosphere. By introducing cooled exhaust gas to the engine 104,the amount of available oxygen for combustion is decreased, therebyreducing combustion flame temperatures and reducing the formation ofnitrogen oxides (e.g., NO_(x)).

Exhaust gas flowing from the donor cylinders 107 to the intake passage114 passes through a heat exchanger such as an EGR cooler 166 to reducea temperature of (e.g., cool) the exhaust gas before the exhaust gasreturns to the intake passage. The EGR cooler 166 may be anair-to-liquid heat exchanger, for example. In such an example, one ormore charge air coolers 132 and 134 disposed in the intake passage 114(e.g., upstream of where the recirculated exhaust gas enters) may beadjusted to further increase cooling of the charge air such that amixture temperature of charge air and exhaust gas is maintained at adesired temperature. In other examples, the EGR system 160 may includean EGR cooler bypass. Alternatively, the EGR system may include an EGRcooler control element. The EGR cooler control element may be actuatedsuch that the flow of exhaust gas through the EGR cooler is reduced;however, in such a configuration, exhaust gas that does not flow throughthe EGR cooler is directed to the exhaust passage 116 rather than theintake passage 114.

Additionally, in some embodiments, the EGR system 160 may include an EGRbypass passage 161 that is configured to divert exhaust from the donorcylinders back to the exhaust passage. The EGR bypass passage 161 may becontrolled via a valve 163. The valve 163 may be configured with aplurality of restriction points such that a variable amount of exhaustis routed to the exhaust, in order to provide a variable amount of EGRto the intake.

In an alternate embodiment shown in FIG. 1, the donor cylinders 107 maybe coupled to an alternate EGR passage 165 (illustrated by the dashedlines) that is configured to selectively route exhaust to the intake orto the exhaust passage. For example, when a second valve 170 is open,exhaust may be routed from the donor cylinders to the EGR cooler 166and/or additional elements prior to being routed to the intake passage114. Further, the alternate EGR system includes a first valve 164disposed between the exhaust passage 116 and the alternate EGR passage165.

The first valve 164 and second valve 170 may be on/off valves controlledby the control unit 180 (for turning the flow of EGR on or off), or theymay control a variable amount of EGR, for example. In some examples, thefirst valve 164 may be actuated such that an EGR amount is reduced(exhaust gas flows from the EGR passage 165 to the exhaust passage 116).In other examples, the first valve 164 may be actuated such that the EGRamount is increased (e.g., exhaust gas flows from the exhaust passage116 to the EGR passage 165). In some embodiments, the alternate EGRsystem may include a plurality of EGR valves or other flow controlelements to control the amount of EGR.

In such a configuration, the first valve 164 is operable to routeexhaust from the donor cylinders to the exhaust passage 116 of theengine 104 and the second valve 170 is operable to route exhaust fromthe donor cylinders to the intake passage 114 of the engine 104. Assuch, the first valve 164 may be referred to as an EGR bypass valve,while the second valve 170 may be referred to as an EGR metering valve.In the embodiment shown in FIG. 1, the first valve 164 and the secondvalve 170 may be engine oil, or hydraulically, actuated valves, forexample, with a shuttle valve (not shown) to modulate the engine oil. Insome examples, the valves may be actuated such that one of the first andsecond valves 164 and 170 is normally open and the other is normallyclosed. In other examples, the first and second valves 164 and 170 maybe pneumatic valves, electric valves, or another suitable valve.

As shown in FIG. 1, the vehicle system 100 further includes an EGR mixer172 which mixes the recirculated exhaust gas with charge air such thatthe exhaust gas may be evenly distributed within the charge air andexhaust gas mixture. In the embodiment depicted in FIG. 1, the EGRsystem 160 is a high-pressure EGR system which routes exhaust gas from alocation upstream of turbochargers 120 and 124 in the exhaust passage116 to a location downstream of turbochargers 120 and 124 in the intakepassage 114. In other embodiments, the vehicle system 100 mayadditionally or alternatively include a low-pressure EGR system whichroutes exhaust gas from downstream of the turbochargers 120 and 124 inthe exhaust passage 116 to a location upstream of the turbochargers 120and 124 in the intake passage 114.

As depicted in FIG. 1, the vehicle system 100 further includes atwo-stage turbocharger with the first turbocharger 120 and the secondturbocharger 124 arranged in series, each of the turbochargers 120 and124 arranged between the intake passage 114 and the exhaust passage 116.The two-stage turbocharger increases air charge of ambient air drawninto the intake passage 114 in order to provide greater charge densityduring combustion to increase power output and/or engine-operatingefficiency. The first turbocharger 120 operates at a relatively lowerpressure, and includes a first turbine 121 which drives a firstcompressor 122. The first turbine 121 and the first compressor 122 aremechanically coupled via a first shaft 123. The first turbocharger maybe referred to the “low-pressure stage” of the turbocharger. The secondturbocharger 124 operates at a relatively higher pressure, and includesa second turbine 125 which drives a second compressor 126. The secondturbocharger may be referred to the “high-pressure stage” of theturbocharger. The second turbine and the second compressor aremechanically coupled via a second shaft 127.

As explained above, the terms “high pressure” and “low pressure” arerelative, meaning that “high” pressure is a pressure higher than a “low”pressure. Conversely, a “low” pressure is a pressure lower than a “high”pressure.

As used herein, “two-stage turbocharger” may generally refer to amulti-stage turbocharger configuration that includes two or moreturbochargers. For example, a two-stage turbocharger may include ahigh-pressure turbocharger and a low-pressure turbocharger arranged inseries, three turbocharger arranged in series, two low pressureturbochargers feeding a high pressure turbocharger, one low pressureturbocharger feeding two high pressure turbochargers, etc. In oneexample, three turbochargers are used in series. In another example,only two turbochargers are used in series.

In the embodiment shown in FIG. 1, the second turbocharger 124 isprovided with a turbine bypass valve 128 which allows exhaust gas tobypass the second turbocharger 124. The turbine bypass valve 128 may beopened, for example, to divert the exhaust gas flow away from the secondturbine 125. In this manner, the rotating speed of the compressor 126,and thus the boost provided by the turbochargers 120, 124 to the engine104 may be regulated during steady state conditions. Additionally, thefirst turbocharger 120 may also be provided with a turbine bypass valve.In other embodiments, only the first turbocharger 120 may be providedwith a turbine bypass valve, or only the second turbocharger 124 may beprovided with a turbine bypass valve. Additionally, the secondturbocharger may be provided with a compressor bypass valve 129, whichallows gas to bypass the second compressor 126 to avoid compressorsurge, for example. In some embodiments, first turbocharger 120 may alsobe provided with a compressor bypass valve, while in other embodiments,only first turbocharger 120 may be provided with a compressor bypassvalve.

According to embodiments disclosed herein, the air flow through thefirst and second turbochargers may be regulated to avoid turbochargerchoke during a transient event. The air flow through the first andsecond turbochargers may be controlled by adjusting the amount of EGRrouted to the intake. For example, if the amount of air flow through thetwo-stage turbocharger exceeds a choke limit for one or both of thestages of the turbocharger, the air flow may be reduced by increasingEGR flow to the intake. Conversely, if the amount of air flow throughthe turbocharger is lower than optimal for performance, or is below asurge limit, the air flow may be increased by reducing the EGR flow tothe intake. The EGR may be adjusted based on a trajectory of EGR amountsthat maintain mass air flow through the turbocharger below respectiveover speed lines (over speed levels) for a plurality of pressure ratiosand turbocharger speeds for each stage of the turbocharger. Additionaldetail regarding adjusting the EGR flow to maintain desired turbochargerair flow will be presented below with respect to FIGS. 2-6.

The vehicle system 100 further includes an exhaust treatment system 130coupled in the exhaust passage in order to reduce regulated emissions.As depicted in FIG. 1, the exhaust gas treatment system 130 is disposeddownstream of the turbine 121 of the first (low pressure) turbocharger120. In other embodiments, an exhaust gas treatment system may beadditionally or alternatively disposed upstream of the firstturbocharger 120. The exhaust gas treatment system 130 may include oneor more components. For example, the exhaust gas treatment system 130may include one or more of a diesel particulate filter (DPF), a dieseloxidation catalyst (DOC), a selective catalytic reduction (SCR)catalyst, a three-way catalyst, a NO_(x) trap, and/or various otheremission control devices or combinations thereof.

The vehicle system 100 further includes the control unit 180, which isprovided and configured to control various components related to thevehicle system 100. In one example, the control unit 180 includes acomputer control system. The control unit 180 further includesnon-transitory, computer readable storage media (not shown) includingcode for enabling on-board monitoring and control of engine operation.The control unit 180, while overseeing control and management of thevehicle system 100, may be configured to receive signals from a varietyof engine sensors, as further elaborated herein, in order to determineoperating parameters and operating conditions, and correspondinglyadjust various engine actuators to control operation of the vehiclesystem 100. For example, the control unit 180 may receive signals fromvarious engine sensors including sensor 181 arranged in the inlet of thehigh-pressure turbine, sensor 182 arranged in the inlet of thelow-pressure turbine, sensor 183 arranged in the inlet of thelow-pressure compressor, and sensor 184 arranged in the inlet of thehigh-pressure compressor. The sensors arranged in the inlets of theturbochargers may detect air temperature and/or pressure. Additionalsensors may include, but are not limited to, engine speed, engine load,boost pressure, ambient pressure, exhaust temperature, exhaust pressure,etc. Correspondingly, the control unit 180 may control the vehiclesystem 100 by sending commands to various components such as tractionmotors, alternator, cylinder valves, throttle, heat exchangers,wastegates or other valves or flow control elements, etc.

Thus, the vehicle system of FIG. 1 may provide for an engine systemcomprising a two-staged turbocharger, an EGR system including a firstEGR valve and a second EGR valve, and a control unit includinginstructions to, during steady state conditions, adjust the first andsecond EGR valves to deliver a first amount of EGR based on intakeoxygen, and in response to a detected transient operating event, adjustthe first and second EGR valves to deliver a second amount of EGR basedon intake oxygen, desired mass air flow through the two-stageturbocharger, and a magnitude of the transient operating event.

The desired mass air flow through the two-stage turbocharger may bebased on a pressure ratio and speed of a first stage of the two-stageturbocharger and a pressure ratio and speed of a second stage of thetwo-stage turbocharger. Additionally or alternatively, the desired massair flow through the two-stage turbocharger may be further based on aposition of a first bypass valve of the first stage and/or a position ofa second bypass valve of the second stage.

The second amount of EGR may be within a range of a third EGR amountthat provides a designated intake oxygen concentration and a fourth EGRamount that provides the designated mass air flow through the two-stageturbocharger, and may be selected from within the range based on themagnitude of the transient operating event.

Thus, according the above-described system, the provided amount of EGRmay be a function of different operating parameters during differentoperating conditions of the engine. During steady state conditions, theprovided EGR amount may be a function of a designated intake oxygenconcentration. However, during a transient operating event, such as anincrease in load, the provided EGR amount may be a function of both thedesignated intake oxygen concentration and a designated mass air flowthrough a turbocharger. During the transient event, the provided amountof EGR may selected from within a range defined by the designated intakeconcentration and the designated mass air flow through the turbochargerbased on a magnitude of the transient event.

FIG. 2 is a flow chart illustrating a method 200 for regulating air flowthrough a turbocharger during a transient event. Method 200 may becarried by an engine control unit, such as control unit 180, accordingto instructions stored thereon. Method 200 may be executed in order toregulate air flow through a multi-stage turbocharger, including at leasttwo turbochargers, such as first turbocharger 120 and secondturbocharger 124. At 202, method 200 includes determining engineoperating parameters. The engine operating parameters may include enginespeed, load, oxygen-fuel ratio (OFR), temperature, and other parameters.Based on the determined parameters, at 204, it is determined if theengine is operating with a transient event. A transient event mayinclude a sudden increase or decrease in engine speed and load, and maybe detected based on engine speed and OFR, or other parameters.

FIG. 3 is a diagram illustrating an embodiment of a control routine 300that may be carried out by the control unit for determining a transientevent and assigning a magnitude value to the transient event. Thetransient event may be detected by comparing engine speed to a referenceengine speed, and additionally or alternatively by comparing OFR to anOFR lower limit. By doing so, the engine's demand for fresh air, whichif it deviates from an expected value indicates a transient operatingevent, may be monitored.

At 302, the current engine speed 304 may be divided by a referenceengine speed 306 to determine an engine speed ratio. The referenceengine speed may be a previously determined engine speed, for examplethe average engine speed during a previous steady state operating event.In other embodiments, the reference engine speed may be an expectedspeed based on current operating conditions, such as an amount of fueldelivered to the engine, engine load, manifold air pressure, etc. Theengine speed ratio is fed into an engine speed look-up table 308. Thelook-up table may assign a transient magnitude value based on themagnitude of the engine speed ratio. For example, as the current enginespeed 304 increases above the reference engine speed 306, the valueassigned by the look-up table may increase. In one example, thetransient magnitude value may be within a range of 0-1.

Similarly, at 310, a reference OFR lower limit 312 is divided by thecurrent OFR 314, and this OFR ratio is fed into an OFR look-up table316. The look-up table may assign a transient magnitude value based onthe magnitude of the OFR ratio. For example, as the OFR increases abovethe OFR lower limit, the ratio may become smaller, and the transientmagnitude value may increase. However, the above described examples arenon-limiting, and the values assigned by the engine speed look-up table308 and the OFR look-up table 316 may be based on any relationship ofthe determined ratios.

At 318, the transient magnitude value determined from the look-up table308 and the transient magnitude value determined from the look-up table316 are compared, and the minimum value between the two is output as thetransient multiplier 320.

Thus, control routine 300 assigns a magnitude value to a transientevent, which may be determined based on engine speed and/or OFR. In someembodiments, if the magnitude value assigned to the transient event isbelow a threshold, then it may be determined that no transient eventoccurred, and that the engine is operating under steady stateconditions. Therefore, returning to FIG. 2, a transient event may bedetected if the magnitude value determined by control routine 300 isabove a threshold. Other mechanisms for determining a transient eventare also possible, such as comparing current engine speed and load to arolling average of engine speed and load.

If a transient event is not detected, method 200 proceeds to 206 toadjust the EGR system valve to deliver an EGR amount for designatedintake oxygen concentration. The designated intake oxygen concentrationmay be an amount of intake oxygen that provides desired combustionconditions, and hence may be based on engine speed and load.Additionally, the designated intake oxygen amount may balance reductionof exhaust constituent emissions, such as NOx emissions, with productionof particulate matter. The amount of EGR routed to the intake impactsthe intake oxygen concentration, due to the EGR, which is low in oxygen,occupying volume otherwise taken up by intake air, which is relativelyhigh in oxygen. Thus, as the EGR amount increases, the intake oxygenconcentration decreases. The designated intake oxygen concentration, andhence the designated amount of EGR to deliver the designated intakeoxygen, may be a function of engine speed, load, OFR, exhaust oxygenfraction, manifold air pressure, manifold air temperature, combustionstability such as an amount of knock, and/or additional parameters. TheEGR valve, which may include valve 163, or in alternate embodiments, theEGR metering valve 170 and EGR bypass valve 164, may be set to a desiredposition to deliver the EGR amount from the donor cylinders to theintake.

At 208, one or more turbocharger valves may be adjusted to maintainoptimal air flow through both the high-pressure stage and thelow-pressure stage of the turbocharger. The optimal air flow through theturbocharger may be based on a desired amount of boost to be provided tothe intake. Additionally, the air flow through the turbocharger may bemaintained under an over speed limit and/or may be adjusted to maintaina desired pressure ratio across the turbocharger turbine and compressor.The turbocharger valves may include a high-pressure turbine bypass valve(e.g., valve 128), high-pressure compressor bypass valve (e.g., valve129), and/or additional turbocharger valves. Method 200 then returns.

Returning to 204 of FIG. 2, if it is determined that the engine isoperating with a transient event, for example if the transient magnitudevalue is above a threshold, method 200 proceeds to 210 to determine afirst EGR amount for designated intake oxygen concentration. The firstEGR amount may be determined similarly to the EGR amount for designatedintake oxygen determined at 206. At 212, a second EGR amount fortransient load acceptance in the multi-stage turbocharger is determined.During a transient increase in engine load, when mass air flow throughthe engine and hence mass air flow through the turbocharger increases,the turbocharger may be subject to air flow amounts above an overspeed/choke limit. To reduce the air flow through the turbocharger, EGRmay be increased. As indicated at 214, the high-pressure stage and thelow-pressure stage of the multi-stage turbocharger each responddifferently to transient events. As demonstrated in examples ofturbocharger flow maps for the high-pressure stage and low pressurestage illustrated in FIGS. 7 and 8, while each stage of the turbochargermay experience the same transient event, the high-pressure stageresponds differently to the transient event than the low-pressure stage.Therefore, the maximum air flow without experiencing choke for eachstage may be determined based on measured turbocharger conditions, andas indicated at 216, the second amount of EGR may be selected in orderto balance optimal flow through both stages and compensate for thedifference in the transient responses between the two stages. Themaximum air flow allowable without experiencing choke in both thehigh-pressure and low-pressure stages of the turbocharger may bedetermined based on determined turbocharger conditions according to thecontrol routines illustrated in FIGS. 4 and 5, which are be presentedbelow. Additional information with respect to the exemplary turbochargerflow maps illustrated in FIGS. 7 and 8 will also be presented below.

At 218, the magnitude of the transient event is determined. Themagnitude of the transient event may be the transient multiplier valueoutput by control routine 300 described above. At 220, the EGR valve(such as valve 163 or in other embodiments valves 164 and 170) isadjusted to deliver an EGR amount to the intake that is within a rangedefined by the first and second EGR amounts. Adjusting the EGR valve mayinclude biasing the EGR amount towards the first or second amount basedon the magnitude of the transient event at 222. The amount of EGRactually delivered to the intake may be based on both the first amountof EGR for providing desired intake oxygen, and the second amount of EGRfor providing maximum turbocharger air flow. Depending on the magnitudeof the transient event, the actual amount of delivered EGR may be withinthe range of the first and second amounts of EGR, but biased towarddelivering a designated intake oxygen amount, for example if thetransient event is relatively small, or toward delivering the maximumturbocharger air flow, for example if the transient event is relativelylarge. The amount of EGR delivered to the intake may provide desiredturbocharger performance while balancing combustion stability andemission reduction. Desired turbocharger performance may includeavoiding turbocharger choke and surge while maintaining air flow throughthe turbocharger in or near a region of relatively high efficiency. Todetermine the amount of EGR to deliver to the intake based on desiredoxygen, turbocharger air flow, and the transient event, the controlroutine depicted in FIG. 6, explained below, may be carried out. Method200 then returns.

Thus, method 200 provides for a method for an engine. The methodincludes, based on a magnitude of a transient operating event, biasingan amount of EGR provided to an intake of the engine toward a first EGRamount that provides designated intake oxygen or toward a second EGRamount that provides designated mass air flow for turbochargerperformance. The designated mass air flow for turbocharger performancemay be a mass air flow for avoiding surge and choke of the turbocharger.The method may also include, if the magnitude of the transient operatingevent is above a threshold, then biasing toward the second EGR amount,and if the magnitude of the transient operating event is below thethreshold, then biasing toward the first EGR amount. The biasing towardthe first or second amount of EGR may include adjusting one or more EGRvalves in order to deliver an amount of EGR to the intake that is withina range of the first and second amounts. The actual amount of deliveredEGR may be biased toward the first EGR amount or toward the second EGRamount based on the magnitude of the transient event, which may bedetermined based on a difference between a reference OFR and a measuredOFR, or between a reference engine speed and a measured engine speed.

The turbocharger may be a two-stage turbocharger, and the designatedmass air flow for performance of the two-stage turbocharger may bewithin a range of a first amount of mass air flow for performance of afirst stage of the two-stage turbocharger and a second amount of massair flow for performance of a second stage of the two-stageturbocharger. The first and second amounts of mass air flow forturbocharger performance may be based on a speed and upstreamtemperature and pressure of the respective turbocharger stages. Thefirst and second amounts of mass air flow for turbocharger performancemay further be based on the transient event response for each respectivestage.

FIGS. 4 and 5 illustrate embodiments of control routines for determiningoptimal air flow through both a high-pressure stage and a low-pressurestage of a two-stage turbocharger. Similar routines may be carried outon any multi-stage turbocharger configuration. The optimal air flowthrough the turbocharger may be based on turbocharger speed. However,the measured turbocharger speed may be corrected based on an airtemperature in order to account for changes in the air density due totemperature.

FIG. 4 is a diagram illustrating a control routine 400 for correcting aturbocharger speed, according to an embodiment. Control routine 400 maycorrect a measured turbocharger speed for a low-pressure stage or for ahigh-pressure stage. As explained above, the turbocharger speed 402 iscorrected based on an engine air temperature 404. The engine airtemperature may be ambient temperature, a temperature of the air at theinlet of the turbocharger (for example, if the speed of the low-pressurestage is being determined), or an interstage temperature (if the speedof the high-pressure stage is being determined).

The engine air temperature 404 may be added to a first constant 406 at408. The first constant 406 may be a value that converts the temperatureto a different scale or unit, or may be a temperature offset. This valueis then divided by a second constant 410 at 412, and then the squareroot is determined at 414. The second constant 410 may be a referencetemperature value, such as the average operating temperature of thatparticular stage of the turbocharger. Thus, the air temperature may betransformed into a value that reflects a degree of deviance from astandard turbocharger operating temperature. At 416, the turbochargerspeed 402 is divided by the output of 414, and this value is output asthe corrected turbocharger speed 418. As a result, the measuredturbocharger speed may be revised to be either increased or decreased inorder to reflect the temperature of the air flowing through theturbocharger, prior to utilizing the turbocharger speed for subsequentair flow calculations.

FIG. 5 is a diagram illustrating an embodiment of a control routine 500for determining optimal turbocharger air flow through high- andlow-pressure stages of the turbocharger. First, to determine optimal airflow through the low-pressure (LP) stage, the barometric pressure 502and inlet air temperature 504 are determined. The barometric pressuremay include the air pressure outside the vehicle in which the two-stageturbocharger is installed, or the barometric pressure may be thepressure of the air flow at the inlet of the LP stage. At 506, thesquare root of the barometric pressure is taken, and at 508 the squareroot of the inlet air temperature is taken. The value output from 506(the square root of the barometric pressure) is then divided by thesquare root of the inlet air temperature (output from 508) at 510. Aproduct of this first value and two additional values, explained below,is determined at 512. The second value is based on the corrected LPturbo speed 514, which may be determined based on the control routine400 described above with respect to FIG. 4. The corrected LPturbocharger speed is entered into a look-up table 516, which outputs anoptimal air flow amount for that turbocharger speed. The look-up table516 may be based on the exemplary turbocharger flow map illustrated inFIG. 8, for example, and the optimal air flow amount determined from thelook-up table may reflect an air flow amount that is outside a chokeregion and a surge region and/or provides desired turbochargerefficiency. Additionally, the optimal air flow amount for the LPturbocharger may be based on the pressure ratio across the turbocharger.

The third value may reflect the current turbocharger stage bias. A stagebias value 518 may reflect the position of a high-pressure turbinebypass valve, for example, and the relative amount of high-pressure airthat is reaching the high-pressure turbine vs. the low-pressure turbine.

At 512, the optimal air flow determined from the look-up table 516 ismultiplied by the stage bias value 518, and this product is divided bythe ratio of the barometric pressure to the inlet temperature, outputfrom 510, to determine a final air flow amount for providing optimallow-pressure flow.

Optimal flow for the high-pressure stage may be determined in a similarmanner. The square root of the interstage temperature 520 and interstagepressure 522 may each be determined at 524 and 526. The interstagetemperature and pressure may be measured between the low-pressureturbocharger and the high-pressure turbocharger. The square root of theinterstage pressure may be divided by the square root of the interstagetemperature at 528 to determine a pressure to temperature ratio. Thecorrected high-pressure (HP) turbocharger speed 532, determined by thecontrol routine 400, may be entered into look-up table 534 to determinean optimal air flow amount for the HP stage, similar to the mechanism bywhich the optimal air flow through the LP stage was determined. Forexample, the optimal air flow through the HP stage may be based oncorrected HP turbocharger speed and turbocharger pressure ratio, and maybe an air flow amount above a surge line but below an over speed orchoke line. The look-up table may be based on the example turbochargerflow map illustrated in FIG. 7, and as such the optimal air flow for thehigh-pressure stage may be different than the optimal air flow for thelow-pressure stage.

This value may be multiplied by a second stage bias value at 530. Thesecond stage bias value may be determined by subtracting the stage biasvalue 518 from a constant 538 at 536. In one example, the stage biasvalue 518 may be on a scale of 0-1, and the constant 538 may be 1. Inthis way, if for example the stage bias value 518 is 0.3, the secondstage bias value may be 0.7, and this may reflect the relativeproportion of the high-pressure air flow that is reaching thelow-pressure stage and the relative proportion that is reaching thehigh-pressure stage.

Further, at 530, the above calculated product may be divided by theinterstage temperature/pressure ratio to output a final high-pressureoptimal air flow amount. The final low-pressure optimal air flow amount,output from 512, and the final high-pressure optimal air flow amountoutput from 530, may be summed at 540. Thus, the mass air flow foroptimal turbocharger performance during the transient event may bedetermined based on a balance between the optimal mass air flows foreach stage of the turbocharger. During an increase in load, the optimalturbocharger mass air flow may be the maximum air flow through theturbocharger before a choke event occurs. The second EGR amount thatprovides the maximum turbocharger air flow, described above with respectto FIG. 2, may be the amount of EGR that reduces the mass air flowthrough the turbocharger to the maximum amount. To determine the EGRamount for providing optimal turbocharger air flow, at 544, the valueoutput from 540 may be subtracted from the total mass flow 542 currentlyflowing through the turbocharger to determine an EGR mass flow for thetransient event at 546.

Turning to FIG. 6, a diagram illustrating an embodiment of a controlroutine 600 for determining an amount of EGR provided to the intake isillustrated. Control routine 600 utilizes the first EGR amount forproviding desired intake oxygen 602, determined based on operatingparameters as described above with respect to FIG. 2, the second EGRamount for providing optimal or maximum turbocharger air flow during thetransient 546, as determined based on the control routine 500 of FIG. 5,and the transient multiplier 320 determined by control routine 300 ofFIG. 3. The transient multiplier 320 is multiplied by the EGR fortransient load acceptance 546 at 604 to determine a final EGR flowamount for transient load acceptance. The transient multiplier 320 isalso subtracted from a constant 606 at 608. Similar to the stage biasdescribed above, the transient multiplier may be on a scale of 0-1, andthe constant may be 1. At 610, the value output from 608 may bemultiplied by the EGR for designated intake oxygen concentration 602 todetermine a final EGR flow amount for designated intake oxygenconcentration. The two final EGR values are summed at 612 to determinethe actual EGR amount mass flow amount 614.

Thus, the control routine 600 selects an EGR amount within a range of afirst possible EGR amount that provides desired intake oxygen and asecond possible EGR amount that provides optimal turbocharger air flowfor accepting the transient load. Depending on the magnitude of thetransient event, the selected EGR amount may be closer to the firstamount or closer to the second amount. As the magnitude of the transientevent increases, higher weight may be given to the second EGR amount forthe turbocharger flow in order to control the transient air flow andavoid turbocharger choke, even if the designated intake oxygenconcentration is not provided. As the magnitude of the transient eventdecreases, higher weight may be given to the first EGR amount for thedesignated intake oxygen. Because the magnitude of the transient eventis small, there is a lower chance for turbocharger choke, and thus theEGR amount may be based mostly on designated intake oxygen concentrationfor controlling exhaust emissions.

FIGS. 7 and 8 are exemplary turbocharger maps for a high-pressure stageand a low-pressure stage of a two-stage turbocharger that illustrate thedifferential response to transient events between the two stages. FIG. 7illustrates a map 700 for a high-pressure stage, and FIG. 8 illustratesa map 800 for a low-pressure stage. For both maps 700 and 800, thepressure ratio across the respective turbocharger is depicted on thevertical axis, while the mass flow through the respective turbochargeris depicted on the horizontal axis.

Referring now to FIG. 7, map 700 includes a plurality of turbochargerspeed lines, for example speed line 702, which are illustrated in solidlines. For each respective speed line, optimal air flow for thehigh-pressure stage may be to the right of the surge line 704 and to theleft of the choke region 706. Within the region of optimal air flow,there are zones of increasing efficiency illustrated by the dashedlines; zone 708 is an example of a highest efficiency zone. Thus, airflow through the high-pressure stage may be controlled to maintain thestage within the high-efficiency zone 708.

However, during a transient event, such as a drop in engine load, theturbocharger may respond by operating within transient zone 710. Asillustrated in FIG. 7, the transient zone 710 for the high-pressurestage is within the boundaries of the surge line 704 and the chokeregion 706. Also illustrated in FIG. 7 is an example response to thehigh-pressure turbine bypass valve being opened. As the turbine bypassvalve redirects air flow around the high-pressure stage, thehigh-pressure stage responds with decreasing mass flow and a decreasedpressure ratio, shown by line 712.

In contrast, the low-pressure stage responds differently to a transientevent than the high-pressure stage. Map 800 of FIG. 8 includes aplurality of turbocharger speed lines 802, a surge line 804, and chokeregion 806, similar to the high-pressure map 700. Map 800 also includeszones of varying efficiency, such as high-efficiency zone 808. Theefficiency zones of the low-pressure map as well as the surge line 804are different in the low-pressure map as compared to the high-pressuremap.

Furthermore, in response to a transient drop in engine load (and furtherin response to the change in EGR flow amounts that may accompany thetransient event), the low-pressure stage operates within transient zone810, which as illustrated in FIG. 8 includes area to the left of thesurge line 804. Furthermore, an example response to the turbine bypassvalve of the high-pressure stage being opened is also illustrated inFIG. 8. Unlike the response illustrated by line 712 of FIG. 7, thelow-pressure stage is pushed towards the surge line as the turbinebypass valve of the high-pressure stage is opened, as illustrated byline 812.

Thus, the low-pressure turbocharger may operate under surge conditionsduring a transient drop in load. Additionally, by opening the turbinebypass valve of the high-pressure turbocharger, the low-pressureturbocharger may be pushed to operate under surge conditions. WhileFIGS. 7 and 8 illustrate a response to a drop in load, an increase inload may also affect the high-pressure turbocharger and the low-pressureturbocharger differentially. For example, in response to an increase inengine load, the low-pressure stage may be pushed across the boundaryinto the choke region, while the high-pressure stage may continue tooperate outside of the choke region. Therefore, the optimal air flowamounts for each stage may be different.

During a transient increase in engine load, for example, typically theamount of EGR routed to the intake of the engine may be reduced. Asdemonstrated above, the low-pressure turbocharger and the high-pressureturbocharger may respond differentially to this transient event. Thus,to prevent the low-pressure turbocharger from operating within the chokeregion described above, the amount of EGR routed to the intake may beadjusted. In one example, depending on turbocharger conditions includingtemperature, speed, pressure ratio, and mass air flow, an EGR amount maybe selected from a trajectory of EGR amounts that are alignedsubstantially parallel to a choke or over speed line of the low-pressureturbocharger. These EGR amounts may maintain the turbocharger below thechoke line for a given turbocharger pressure ratio and mass air flow. Inanother example, the EGR amount routed to the intake may be adjusted toprovide a balance between a designated intake oxygen concentration thatprovides stable combustion conditions and controls engine out emissions,and a designated system mass air flow that provides designated flowthrough both the high-pressure and low-pressure turbochargers. Thedesignated flow may be an amount that avoids choke and surge and/or thatmaintains relatively high turbocharger efficiency.

Thus, the systems and methods described herein provide for a method foran engine. The method may include in response to a transient engineoperating event, adjusting a provided amount of exhaust gasrecirculation (EGR) provided to an intake of the engine based on atleast one of designated intake oxygen concentration, one or moreturbocharger conditions, and a magnitude of the transient engineoperating event.

The method may include determining a magnitude of a transient engineoperating event based on a change in engine speed or other parameters.The turbocharger conditions may be determined based on informationreceived from one or more sensors. The provided amount of EGR providedto the intake may be adjusted responsive to the transient operatingevent to provide one or more of a first EGR amount for the designatedintake oxygen concentration and a second EGR amount for a designatedmass air flow, the designated mass air flow determined based on theturbocharger conditions.

Another embodiment relates to a method for an engine. The methodcomprises, in response to a transient engine operating event, adjustinga provided amount of exhaust gas recirculation (EGR) provided to anintake of the engine based on at least one of designated intake oxygenconcentration, one or more turbocharger conditions of a turbocharger,and/or a magnitude of the transient engine operating event. In anotherembodiment, the provided amount of EGR is adjusted based on at least twoof the designated intake oxygen concentration, the one or moreturbocharger conditions of a turbocharger, and/or the magnitude of thetransient engine operating event. In another embodiment, the providedamount of EGR is adjusted based on all of the designated intake oxygenconcentration, the one or more turbocharger conditions of aturbocharger, and the magnitude of the transient engine operating event.

Another embodiment relates to a method for an engine. The methodcomprises, in response to a transient engine operating event, adjustinga provided amount of exhaust gas recirculation (EGR) provided to anintake of the engine based on one or more turbocharger conditions of atwo-stage turbocharger coupled to the engine and having at least firstand second turbocharger stages. (In other embodiments, the providedamount is adjusted further based on designated intake oxygenconcentration and/or a magnitude of the transient engine operatingevent.) The turbocharger conditions comprise one or more of temperature,mass air flow, pressure ratio, speed, or one or more transient responsesof the first and second turbocharger stages.

Another embodiment relates to a method for an engine. The methodcomprises detecting a transient engine operating event based on adifference between engine speed and a reference engine speed. The methodfurther comprises determining a magnitude of the transient engineoperating event based on a degree of the difference. The method furthercomprises, in response to detecting the transient engine operatingevent, adjusting a provided amount of exhaust gas recirculation (EGR)provided to an intake of the engine based on the magnitude of thetransient engine operating event. (In other embodiments, the providedamount is adjusted further based on at least one of designated intakeoxygen concentration and/or one or more turbocharger conditions of aturbocharger.)

As explained above, the terms “high pressure” and “low pressure” arerelative, meaning that “high” pressure is a pressure higher than a “low”pressure. Conversely, a “low” pressure is a pressure lower than a “high”pressure.

As used herein, an element or step recited in the singular and proceededwith the word “a” or “an” should be understood as not excluding pluralof said elements or steps, unless such exclusion is explicitly stated.Furthermore, references to “one embodiment” of the present invention arenot intended to be interpreted as excluding the existence of additionalembodiments that also incorporate the recited features. Moreover, unlessexplicitly stated to the contrary, embodiments “comprising,”“including,” or “having” an element or a plurality of elements having aparticular property may include additional such elements not having thatproperty. The terms “including” and “in which” are used as theplain-language equivalents of the respective terms “comprising” and“wherein.” Moreover, the terms “first,” “second,” and “third,” etc. areused merely as labels, and are not intended to impose numericalrequirements or a particular positional order on their objects.

This written description uses examples to disclose the invention,including the best mode, and also to enable a person of ordinary skillin the relevant art to practice the invention, including making andusing any devices or systems and performing any incorporated methods.The patentable scope of the invention is defined by the claims, and mayinclude other examples that occur to those of ordinary skill in the art.Such other examples are intended to be within the scope of the claims ifthey have structural elements that do not differ from the literallanguage of the claims, or if they include equivalent structuralelements with insubstantial differences from the literal languages ofthe claims.

1. A method for an engine, comprising: in response to a transient engineoperating event, adjusting a provided amount of exhaust gasrecirculation (EGR) provided to an intake of the engine based on atleast one of designated intake oxygen concentration, one or moreturbocharger conditions of a turbocharger, or a magnitude of thetransient engine operating event.
 2. The method of claim 1, wherein theturbocharger is a two-stage turbocharger having at least first andsecond turbocharger stages, and wherein the turbocharger conditionscomprise one or more of temperature, mass air flow, pressure ratio,speed, or one or more transient responses of the first and secondturbocharger stages.
 3. The method of claim 2, wherein the one or moretransient responses differ for the first and second turbocharger stages,and wherein the provided amount of EGR is adjusted to compensate for thedifferent transient responses.
 4. The method of claim 2, whereinadjusting the provided amount of EGR provided to the intake of theengine based on one or more of the designated intake oxygenconcentration, one or more turbocharger conditions, or the magnitude ofthe transient engine operating event further comprises: determining afirst determined amount of EGR for providing the designated intakeoxygen concentration; determining a second determined amount of EGR forproviding an air flow amount through the two-stage turbocharger thatbalances maximum air flow for a measured pressure ratio and measuredturbocharger speed for the first turbocharger stage and maximum air flowfor a measured pressure ratio and measured turbocharger speed for thesecond turbocharger stage; and based on the magnitude of the transientengine operating event, adjusting the provided EGR amount to within arange of the first and second determined amounts of EGR.
 5. The methodof claim 4, further comprising correcting the measured turbochargerspeeds for the first and second turbocharger stages based on intake airtemperature.
 6. The method of claim 4, wherein determining the seconddetermined amount of EGR further comprises determining the seconddetermined amount of EGR based on a trajectory of EGR amounts thatmaintain mass air flow through the first and second turbocharger stagesbelow respective over speed levels for a plurality of pressure ratiosand turbocharger speeds.
 7. The method of claim 4, wherein the firstdetermined amount of EGR is determined based on exhaust constituentlevels, oxygen-fuel ratio, and combustion stability, and furthercomprising, biasing the provided amount of EGR toward the seconddetermined amount of EGR as the magnitude of the transient eventincreases.
 8. The method of claim 1, wherein the transient engineoperating event is detected based on a difference between engine speedand a reference engine speed, and wherein the magnitude of the transientengine operating event is based on a degree of the difference.
 9. Amethod for an engine, comprising: biasing, based on a magnitude of atransient operating event, a provided amount of exhaust gasrecirculation (EGR) provided to an intake of the engine toward a firstEGR amount that provides designated intake oxygen concentration ortoward a second EGR amount that provides desired mass air flow forturbocharger performance of a turbocharger coupled to the engine. 10.The method of claim 9, wherein biasing the provided amount of EGR towardthe second EGR amount further comprises biasing the provided amount ofEGR toward the second EGR amount that provides a designated mass airflow based on at least turbocharger pressure ratio and speed.
 11. Themethod of claim 9, wherein biasing the provided amount of EGR toward thefirst EGR amount or toward the second EGR amount further comprises, ifthe magnitude of the transient operating event is above a threshold,then biasing toward the second EGR amount, and if the magnitude of thetransient operating event is below the threshold, then biasing towardthe first EGR amount.
 12. The method of claim 9, wherein theturbocharger is a two-stage turbocharger and the desired mass air flowfor performance of the two-stage turbocharger is within a range definedby a first amount of mass air flow for performance of a first stage ofthe two-stage turbocharger and a second amount of mass air flow forperformance of a second stage of the two-stage turbocharger.
 13. Themethod of claim 12, further comprising determining the first amount ofmass air flow based on interstage temperature and pressure and a speedof the first stage of the two-stage turbocharger.
 14. The method ofclaim 13, further comprising determining the second amount of mass airflow based on barometric pressure, air inlet temperature, and a speed ofthe second stage of the two-stage turbocharger.
 15. An engine system,comprising: an engine; a multi-stage turbocharger coupled to the engine;an exhaust gas recirculation (EGR) system including a first EGR valveand a second EGR valve; and a control unit configured to: adjust thefirst and second EGR valves to deliver a first amount of EGR based onintake oxygen concentration during steady state conditions; and adjustthe first and second EGR valves in response to a detected transientoperating event to deliver a second amount of EGR based on intake oxygenconcentration, designated mass air flow through the multi-stageturbocharger, and a magnitude of the transient operating event.
 16. Theengine system of claim 15, wherein the transient operating eventcomprises an increase in engine load.
 17. The engine system of claim 15,wherein the designated mass air flow through the multi-stageturbocharger is based on a pressure ratio and speed of a first stage ofthe multi-stage turbocharger and a pressure ratio and speed of a secondstage of the multi-stage turbocharger.
 18. The engine system of claim17, wherein the designated mass air flow through the multi-stageturbocharger is further based on a position of a first bypass valve ofthe first stage and a position of a second bypass valve of the secondstage.
 19. The engine system of claim 17, wherein the second amount ofEGR is within a range defined by a third EGR amount that provides adesignated intake oxygen concentration and a fourth EGR amount thatprovides the designated mass air flow through the multi-stageturbocharger.
 20. The engine system of claim 19, wherein the secondamount of EGR is selected from within the range based on the magnitudeof the transient operating event.